In a paper pulping process, wood chips are treated in a digester system to separate the cellulose fibers and to remove lignin, which binds the fibers together in the natural state of wood. Digestion of wood chips using heat and chemicals is a common practice in the industry. In the so called “kraft process”, wood chips and an alkaline digesting liquor are introduced to a digester creating pulp and black liquor, which is a lignin containing soluble part. After the digestion process the produced pulp and the black liquor are separated. The pulp comprises cellulose fibers and is typically treated further to make paper. The black liquor comprises lignin, hemicelluloses, inorganic salts, and other extractive components. The black liquor can be withdrawn from the digester and can be treated further by burning it in specific recovery boilers for energy production and recovering of the salt, which can be used again in the kraft process. Some part of the black liquor can be used to recover lignin. Lignin can subsequently be depolymerized to the corresponding alkylphenol (ethers), which can react further and be used for several applications.
There are several other ways of processing black liquor known in the art. Black liquor contains energetically valuable big aromatic structures, such as lignin, and processing the black liquor is performed to obtain energy. Depending on the processing method of the black liquor, besides energy, different compositions of compounds or different concentrations of the compositions are further obtained. Black liquor gasification, using air or oxygen as the gasification medium is used to obtaining compounds such as syngas. The syngas can be further processed to obtain bio-fuel for automotive and other industrial uses or can be catalytically converted to MeOH, dimethyl ether, or other higher molecular weight and more valuable organic molecules.
Though treated black liquor has found applications in various markets, there is still a need for providing other ways of processing black liquor. A lot of research has been conducted by treating the black liquor to recover the salt by other ways than by using the recovery boiler. A disadvantage is that parts of the existing pulp mills installation become redundant.